The present invention relates to limited slip differentials, and more particularly to limited slip differentials having an electromagnetically actuated clutch.
Differentials are well known in the prior art and allow each of a pair of output shafts or axles operatively coupled to a rotating input shaft to rotate at different speeds, thereby allowing the wheel associated with each output shaft to maintain traction with the road while the vehicle is turning. Such a device essentially distributes the torque provided by the input shaft between the output shafts.
The completely open differential, i.e., a differential without clutches or springs which restrict relative rotation between the axles and the rotating differential casing, is suitable, and even preferable, for normal, dry driving conditions, but is not well suited to slippery conditions in which one driven wheel experiences a much lower coefficient of friction than the other driven wheel: for instance, when one wheel of a vehicle is located on a patch of ice and the other wheel is on dry pavement. Under such conditions, the wheel experiencing the lower coefficient of friction loses traction and a small amount of torque to that wheel will cause a xe2x80x9cspin outxe2x80x9d of that wheel. Since the maximum amount of torque which can be developed on the wheel with traction is equal to torque on the wheel without traction, i.e. the slipping wheel, the engine is unable to develop any torque and the wheel with traction is unable to rotate. A number of methods have been developed to limit wheel slippage under such conditions.
Prior means for limiting slippage between the axles and the differential casing use a frictional clutch mechanism, clutch plates and/or a frustoconical engagement structure, operatively located between the rotating casing and the axles. Certain embodiments of such prior means provide a clutch element attached to each of the side gears, and which frictionally engages a mating clutch element attached to the rotating casing or, if the clutch is of the conical variety, a complementary interior surface of the casing itself. Such embodiments may also include a bias mechanism, usually a spring, to apply an initial preload between the clutch and the differential casing. By using a frictional clutch with an initial preload, a minimum amount of torque is always applied to a wheel having traction, e.g., a wheel located on dry pavement. The preload allows the clutch to be more readily fully engaged and generally provides limited slip differential which is more quickly responsive to demands for increased traction.
Limited slip differentials often employ clutches which move, and may become at least partially engaged or preloaded, in response to axial movement of the side gears. Usually two side gears are disposed internal to the differential casing and are each rotatably fixed to one of the two axle shafts. Axial movement of the side gears typically results in response to gear separating forces acting between the pinion gears which revolve about the casing""s axis of rotation and the side gears intermeshed therewith. The gear separating forces urge the two side gears axially outward, away from each other, causing the clutch to lightly engage and develop additional torque at the driven wheels. Examples of such limited slip differentials which comprise cone clutches are disclosed in U.S. Pat. Nos. 4,612,825 (Engle), 5,226,861 (Engle) and 5,556,344 (Fox), each of which is assigned to Auburn Gear, Inc., the disclosures of which are all expressly incorporated herein by reference.
Such differentials have certain amount of internal drag during cornering on dry pavement, when the axle is unlocked and relative rotation occurs between the rotating casing and the axles. This drag, although facilitating faster locking of the differential, is somewhat undesirable in that it results in vehicle inefficiencies and possibly higher temperatures within the differential casing or axle housing, and causes differential component wear.
Certain embodiments of such limited slip differentials utilize an electromagnet having a wire coil to effect the initiating force and actuate the clutch, as disclosed in U.S. Pat. Nos. 5,989,147 (Forrest et al.), 6,019,694 (Forrest et al.), and 6,165,095 (Till et al.), each of which is assigned to Auburn Gear, Inc., the disclosures of which are all expressly incorporated herein by reference. Each of these references discloses an electromagnet which, when selectively energized, actuates a clutch within the differential. The electromagnet is mounted in fixed relationship to the axle housing and is rotatably supported by the differential casing. Alternatively, as disclosed in U.S. Pat. No. 6,309,320 (Forrest et al.), which is assigned to Auburn Gear, Inc., the disclosure of which is expressly incorporated herein by reference, the electromagnet may be fixedly supported by the axle housing. In either case, activation of the electromagnet results in the axle being rotatably locked to the rotating differential casing through the clutch. Relative to some other types of limited slip differentials, those having electromagnetically-actuated clutches enjoy the advantages of variable and/or selective engagement, often at a lower cost.
Cone clutches are generally better suited than disc-type clutches as the clutch elements brought into engagement electromagnetically owing to their unitary, ferrous structure, which provides a superior flux path. However, the load carrying capability of a cone clutch is limited, for a given axial engagement force, by the magnitude of the included angle formed by its engagement surfaces. Typically, these angles range from 9xc2x0 to 12.5xc2x0. The smaller this angle, the greater the torque capacity of the cone clutch. The smaller this angle, however, the harsher the clutch engagement, and the lesser the tendency for the clutch to release. On the other hand, clutches having multiple interleaved discs or plates, or xe2x80x9cclutch packs,xe2x80x9d which are well known in the art, generally have greater torque capacity than a cone clutch of approximately equal package size. Moreover, the required tolerances associated with manufacturing disc clutches tend to be somewhat looser than with cone clutches. Clutch packs, however, do not respond as well as cone clutches do in response to a magnetic actuation force.
Some prior art electromagnetically-actuated limited slip differentials employ both cone and plate clutches, thereby enjoying superior actuation performance and load-carrying capabilities. Such a differential is disclosed in U.S. patent application Ser. No. 10/090,666, filed Mar. 5, 2002, and entitled xe2x80x9cElectromagnetically-Actuated Limited Slip Differential,xe2x80x9d the complete disclosure of which is expressly incorporated herein by reference. These differentials, however, still have some preload on at least one of the clutches.
Some prior art electromagnetically-actuated limited slip differentials employ both cone and plate clutches, thereby enjoying superior actuation performance and load-carrying capabilities. Such a differential is disclosed in U.S. patent application Ser. No. 10/090,666, filed Mar. 5, 2002, now U.S. Pat. No. 6,582,336, and entitled xe2x80x9cElectromagnetically-Actuated Limited Slip Differential,xe2x80x9d the complete disclosure of which is expressly incorporated herein by reference. These differentials, however, still have some preload on at least one of the clutches.
It is desirable to provide an electromagnetically actuated locking differential of high torque capacity which, when its coil is deenergized, reverts to being an open differential having no preloaded clutch(es), thereby eliminating the above-mentioned drag internal to the differential and thus improving vehicle efficiency and reducing differential temperatures and component wear.
The present invention provides an electromagnetically actuated locking differential assembly which, when its coil is deenergized, reverts to a open differential having no clutch preload.
The present invention provides a differential assembly including a rotatable casing having an axis of rotation, a selectively energized electromagnet proximal the casing, and a rotatable first clutch disposed within the casing, the first clutch placed in operative engagement with the casing in response to the electromagnet being energized. Relative rotation between the first clutch and the casing is slowed by their being in operative engagement. A rotatable clutch hub and a second clutch are disposed within the casing, and the casing and the clutch hub are rotatably coupled through engagement of the second clutch, which is operatively engaged in response to relative rotation between the first clutch and the casing being slowed. At least one rotatable pinion gear is disposed within the casing and revolves about the axis of rotation. At least one side gear engaged with the pinion gear and rotatable about the axis of rotation is disposed within the casing, and is rotatably coupled to the casing through the second clutch during engagement of the second clutch. Neither of the first clutch and the second clutch is appreciably engaged with the casing in response to gear separating forces exerted between the pinion gear and the side gear.
The present invention also provides a differential assembly including a rotatable casing having an axis of rotation, a selectively energized electromagnet proximal the casing, and rotatable first and second clutches disposed within the casing and placed in operative engagement with the casing during times when the electromagnet is energized. Disposed within the casing are a rotatable pinion gear revolving about the axis of rotation, and a side gear enmeshed with the pinion gear and rotatable about the axis of rotation, the side gear being rotatably coupled to the casing through the second clutch during engagement of the second clutch. Means are also provided for isolating the first and second clutches from gear separating forces exerted between the pinion gear and the side gear, and preventing appreciable engagement of the first and second clutches with the casing during times when the electromagnet is not energized.